The defense contractor told police in early August that the voices were harassing and following him. In the following weeks, Aaron Alexis moved on to the Washington area for work, and law enforcement officials say he was being treated for serious mental problems. But his security clearance was not stripped, and the 34-year-old Alexis entered the sprawling installation Monday with a valid pass.

A picture of his path and what happened in the 30-plus-minute rampage emerged the day after the shooting that killed 12 victims and injured eight. But his motive remained a mystery, and other questions were left unanswered. Investigators have found no manifesto or other writings suggesting a political or religious motivation for Monday's massacre.

Meanwhile, attack has prompted at least three investigations into government security and background screenings.

The secretary of the Navy is ordering a pair of security reviews -- focusing on how well the Navy protects its bases, and how accurately it screens its workers. And President Barack Obama has ordered his budget office to examine security standards for government contractors and employees across federal agencies.